The present invention relates to ceramic dielectri compositions which have dielectric constants between 2400 and 3000; low dissipation factor (DF), e.g. below 2%; high insulation resistance (R) capacitance (C) products (RC), e.g. above about 5000 ohm-farad at 25.degree. C., above about 1000 ohm-farad at 125.degree. C.; and stable temperature coefficient (TC), in which the dielectric constant does not alter from its base value at 25.degree. C. by more than 15% over a temperature range from -55.degree. C. to 125.degree. C. In addition, the dielectric powders have an average particle size of less than 0.8 microns.
The ceramic compositions of this invention are useful in manufacturing multilayer ceramic capacitors (hereinafter MLC) which require a high capacitance and which typically have a relatively small size. MLC's are commonly made by casting or otherwise forming insulating layers of dielectric ceramic powder upon which conducting metal electrode layers, usually consisting of a palladium/silver alloy, are placed. Firing the material at temperature greater than or equal to 1280.degree. C. is required to form the MLC device. Pure barium titanate (BaTiO.sub.3) has a dielectric constant that is relatively unchanging with temperature changes except for a large spike at 125.degree. C. The dielectric constant at this temperature tends to be as much as an order of magnitude greater than at room temperature. Downward Curie temperature shifters such as strontium, niobium, zirconium and lanthanum are commonly used to move the Curie point toward a more desirable temperature (e.g. 25.degree. C.) where a high dielectric constant is required. The stability of the dielectric constant over a wide range of temperatures, and its insulation resistance, are important factors to be considered in selecting ceramic compositions for use in MLC's. For example, insulation resistance may vary substantially based on grain sizes after final sintering.
It is well known that a temperature stable MLC can be produced by firing barium titanate together with minor oxide additives for control of the final dielectric properties. In a desirable dielectric ceramic compositions for MLC applications which require stability in the dielectric constant over a wider temperature range, the dielectric constant will not change more than plus or minus 15% from its reference value at approximately room temperature (25.degree. C.). The insulation resistance and capacitance product of such compositions should be more than 1000 ohm-farad at 25.degree. C. and more from 100 ohm-farad at the maximum working temperature, usually 125.degree. C., which are requirements in most industry specifications (e.g. EIA-RS198C).
Barium titanate which is used for commercial applications is at present physically characterized by a particle size distribution where 50% or greater of the particles are larger than 1 micron in diameter. It is well known that this can be achieved through air impact milling. This physical feature limits how thin an insulating layer or ceramic dielectric can be in a multilayer capacitor. Since MLC technology looks toward still more miniaturization in devices, it is more desirable to use a ceramic dielectric which is 26 physically characterized by a particle size distribution where 50% or greater of the particles are finer than 0.8 microns in diameter; has flat TC characteristics and in which the dielectric constant does not vary more than 15% from the base value at 25.degree. C.; has a dielectric constant between 2400 and 3000; and the RC product at 25.degree. C. is about 5000 ohm-farads, and about 1000 ohm-farads at 125.degree. C.
However, ceramic compositions as disclosed in prior arts, such at U.S. Pat. Nos. 4,882,305 and 4,816,430 lose their property stability, especially TC characteristics, as the particle size of the powder is reduced to less than 0.8 microns. A typical example is given in the following. Without changing its ceramic composition, particle size of a typical dielectric material having a dielectric constant of about 3000 and less than plus or minus 15% TC is reduced from about 1.1 microns to 0.9, 0.8, 0.7, and 0.5 microns. The dielectric properties as shown below indicates that when the average particle size is less than 0.8 microns, the TC can no longer meet the plus or minus 15% requirements.
______________________________________ Average 0.9 0.8 0.8 0.7 0.5 particle size (.mu.m) Dielectric 3050 3310 3495 4130 5011 constant TC (%) at -55.degree. C. -10.4 -5.3 -3.4 -8.9 -45.8 25.degree. C. 0 0 0 0 0 85.degree. C. -8.2 -8.9 -9.2 -17.2 -30.1 105.degree. C. -6.8 -8.2 -8.3 -19.2 -40.9 125.degree. C. 3.3 2.0 -0.3 -15.4 -46.0 ______________________________________